The present invention relates to the use of a gas mixture consisting solely of helium and of nitrogen in a method of laser welding tailored blanks intended to form parts of motor vehicles, in particular of tailored blanks having to be pressed after welding.
Laser beam welding is a very high performance assembly method since it makes it possible to obtain, at high speeds, penetration depths which are very great compared to other more conventional methods, such as plasma welding, MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.
This is explained by the high power densities used when focussing, by means of one or more mirrors or lenses, the laser beam on the junction plane of the parts to be welded, for example power densities which may exceed 106 W/cm2.
These high power densities cause high vaporization on the surface of the parts which, on expanding outwards, causes progressive hollowing of the weld pool and leads to the formation of a narrow and deep vapour capillary, called a keyhole, in the thickness of the metal sheet, that is to say in the joint plane.
This capillary allows the energy from the laser beam to be deposited directly deep within the metal sheet, this being achieved in contrast to the more conventional welding methods where the deposition of energy is localized on the surface, as in TIG, or even MIG, welding.
For this purpose, the following documents may be mentioned: DE-A-2713904, DE-A-4034745, JP-A-01048692, JP-A-56122690, WO 97/34730, JP-A-01005692, DE-A-4123716, JP-A-02030389, U.S. Pat. No. 4,871,897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693, JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553, WO-A-98/14302, DE-A-3619513 and DE-A-3934920.
This capillary consists of a mixture of metal vapours and of a plasma of metal vapours which have the particular property of absorbing the laser beam and of trapping the energy within the actual capillary.
One of the problems of laser welding is the formation of a covering gas plasma.
More specifically, the metal vapour plasma, by seeding the covering gas or shielding gas with free electrons, may trigger the appearance of a covering gas plasma which is detrimental to the welding operation.
The incident laser beam may then be strongly perturbed by the covering gas plasma.
The interaction of the covering gas plasma with the laser beam may take various forms but, most often, the result is that of absorbing and/or of diffracting the incident laser beam which may lead to a significant reduction in the effective laser power density on the target surface, leading to a decrease in the penetration depth, or even a loss of coupling between the beam and the material, and therefore to a momentary interruption in the welding process.
The power density threshold, above which the plasma appears, depends on the ionization potential of the covering gas used and is inversely proportional to the square of the wavelength of the laser beam.
Thus, it is very difficult to weld under pure argon with a laser of the CO2 type, while this operation can be carried out with far fewer problems using a laser of the YAG type.
Generally, with CO2 laser welding, helium is used as a covering gas, which is a gas with a high ionization potential and which makes it possible to protect against the appearance of the covering gas plasma up to a laser power of at least 45 kW.
However, helium has the drawback of being an expensive gas and many laser users would prefer to use other gases or gas mixtures which are less expensive than helium but which nevertheless would make it possible to limit the appearance of the covering gas plasma and therefore to obtain welding results similar to those obtained with helium but at lower cost.
Thus, commercially there are gas mixtures containing argon and helium, for example the gas mixture containing 30% helium by volume, the rest being argon, marketed under the name LASA(trademark) 2045 by L""AIR LIQUIDE(trademark), which gases make it possible to obtain substantially the same results as helium, for CO2 laser powers less than 5 kW and provided that the power densities generated are not too high, that is to say less than about 2000 kW/cm2.
However, the problem which occurs with this type of Ar/He mixture is that, for higher laser power densities, it is no longer suitable since the threshold for creating the shielding gas plasma is then exceeded.
Moreover, another known problem relates to the economic aspect of welding tailored blanks intended for the automotive field.
Specifically, the requirements of motor vehicle manufacturers for welding tailored blanks are so strict that the investment needed for producing these welds is very high.
The tailored blanks are parts forming motor vehicles which must be assembled by full penetration welding, that is to say that the weld must pass completely through the thickness of the welded part.
Given their usual low thicknesses, typically less than 3 mm, and the high welding speeds set by the motor vehicle manufacturer ( greater than 7 m/min), it is known to assemble the tailored blanks by laser welding.
Since laser welding makes it possible to obtain narrow and deep weld beads, the regions affected thermally by this welding method remain localized close to the region of interaction with the laser, limiting the dimensions of the region where the zinc surface coating is evaporated.
Laser welding of the tailored blanks requires the use of a shielding gas, which is generally helium.
The aim of the present invention is therefore to provide a laser welding method using a gas welding mixture suitable for welding thin tailored blanks, possibly having a zinc coating, and subsequently having to be shaped, especially pressed, which gas leads to forming a cooler metal plasma for welding so as to obtain a strong weld joint, that is to say which is able to withstand one or more subsequent pressing steps without breaking, and which is much less expensive than a welding method using pure helium.
The solution of the invention is therefore a method of welding tailored blanks consisting of a material mostly containing steel and a proportion greater than 0.015% aluminium by weight, using at least one laser beam, in which at least one weld joint of the full-penetration type is made using a gas mixture consisting of 30% to 80% nitrogen by volume, the rest being helium (up to 100%) in order to assist the said laser beam.
Within the scope of the invention, the term xe2x80x9cweld joint of the full-penetration typexe2x80x9d is understood to mean that the joint involves the entire thickness of the metal sheets to be assembled and that part of the laser beam passes through the parts or, in other words, fully penetrates to the other side of the weld bead.
The minimum aluminium content of 0.015% in the basic metal makes it possible to limit the breaking problems occurring after to welding, in particular during the steps of shaping the parts welded in this way, when the latter are made of high yield strength (HYS) steel, for example when deforming the welded parts by pressing.
In fact, the nitrogen contained in the metal in its xe2x80x9cfreexe2x80x9d form is one of the chemical elements responsible for this effect. It contributes to the xe2x80x9cageingxe2x80x9d of the steels by decreasing their deformability, which is a serious problem for metal sheets which have to be pressed.
To solve this problem, the aluminium content of the basic steel is increased so as to fix the nitrogen by promoting the formation of aluminium nitride. The nitrogen fixed in this way by the aluminium is no longer detrimental during the abovementioned deformation steps.
As a result, during the welding process, the use of shielding gas mixtures containing nitrogen mixed with helium does not lead to a detrimental effect on the quality of the weld as the aluminium content of the welded steel is enough to fix the quantities of nitrogen which are introduced into the material.
The method of the invention may comprise one or more of the following technical characteristics, as appropriate:
the basic material of the welded metal sheets is a conventional pressing steel defined according to the EN10130 standard, a pressing steel with a high yield strength (HYS) having an elastic limit greater than 300 MPa and defined according to the EN10149 standard, or a single alloy steel according to the NFA36232 standard.
the assist gas contains from 40 to 70% nitrogen by volume, preferably from 45 to 60% nitrogen.
the laser is of the CO2 type and has a power of from 4 to 10 kW, preferably from 6 to 8 kW.
the laser is of the YAG type and has a power of 1 to 6 kW.
the tailored blanks to be welded have a thickness of between 0.4 and 3 mm.
the tailored blanks have a metal surface coating which improves their corrosion resistance, in particular a zinc surface coating obtained by galvanizing or electrogalvanizing.
the tailored blanks to be welded are placed end to end, then welded.
after welding, the welded blanks are subjected to at least one shaping step.
after welding, the welded blanks undergo at least one pressing step. A pressing operation consists in giving a flat metal sheet or a flat tailored blank a three-dimensional shape obtained by deforming the metal sheet or the tailored blank by means of a press whose matrices reproduce the shape to be obtained.
the tailored blanks consist of a material of the same grade and different thickness or else different grades and identical or different thicknesses.
the welding is done by means of mono or multi-spot (impact) focal spot.
the focal spot is circular or oblong.
the gas flow rate is between 5 l/min and 100 l/min.
the gas pressure is between 1 and 5 bar.
the nozzle dispensing the gas is a lateral or axial nozzle having a diameter ranging from 3 to 30 mm.
According to another aspect, the invention also relates to a method of manufacturing a motor vehicle, in which a method of laser welding tailored blanks according to the invention is used.
The invention will be better understood by means of the examples illustrated below and the appended figures.